1. Field of the Invention
The invention relates generally to device manufacturing by means of a lithographic method and in particular the exposure of radiation sensitive layers by means of charged particles, in particular by means of a beam of electrons.
2. Background Art
The devices manufactured by means of a lithographic method are primarily highly miniaturized devices, such as micro-mechanical structures or integrated circuits (ICs), for example. As far as integrated circuits are concerned, a mask, also referred to as a reticle, includes a circuit pattern which corresponds to one single layer of the circuit to be formed on a suitable substrate, for example, a silicon wafer. In order to image the pattern onto a target area, also referred to as a die, of the substrate, the substrate is first covered with a radiation sensitive layer, also referred to as a resist. Subsequently, the radiation sensitive layer is exposed or irradiated so that the pattern of the mask is imaged onto the radiation sensitive layer by means of charged particles. The radiation sensitive layer is then developed and either the irradiated or exposed regions or the non-irradiated or unexposed regions of the exposed layer are removed. The remaining structure of the radiation sensitive layer is then used as a mask, for example, in an etching step, an ion implantation step, a material deposition step or the like.
Charged particles, for example, electrons or ions, are used for this purpose to enable the formation of such small structures that cannot be produced or only with great difficulties by means of conventional photo-optical imaging process due to the diffraction limitation involved therewith.
The SCALPEL process (Scattering with Angular Limitation in Projection Electron-beam Lithography) is known as a process which employs a beam of electrons for exposing the radiation sensitive layer. This process is described in the article “SCALPEL: A Projection Electron-Beam Approach to Sub-Optical Lithography”, Technology Review, December 1999, by J. A. Liddle, Lloyd R. Harriott, A. E. Novembre and W. K. Waskiewicz, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, N.J. 07974, USA. The disclosure of said document is incorporated herein by reference in its entirety. Furthermore, U.S. Pat. Nos. 5,079,112; 5,130,213; 5,260,151; 5,376,505; 5,258,246; 5,316,879; as well as European Patent Application Numbers 0,953,876 A2 and 0,969,326 A2 relate to the SCALPEL process. The disclosures of the above-mentioned patents and patent applications are incorporated herein by reference in their entirety.
The SCALPEL process is described below with reference to Prior Art FIG. 1.
FIG. 1 schematically shows a mask 1 suitable for the exposure of a radiation sensitive layer on a substrate 3 by means of an electron beam. The mask includes supporting struts 7 to which a membrane layer 5 is attached which is relatively transparent for the electrons. The pattern to be imaged onto the substrate 3 is formed by scattering regions 9 provided on the relatively transparent membrane layer. Electron beams 11 and 12 passing only through the membrane layer 5 and not through the scattering regions 9 pass through the mask 1 substantially unscattered and substantially straight or at relatively small scattering angles, whereas electron beams 13 which pass through the membrane layer 5 and the scattering regions 9 are deflected from their original direction by a larger scattering angle in the scattering regions 9. The electron beams 11, 12, 13 pass through an electromagnetic or/and electrostatic projection lens system 15 and are deflected by the same such that the unscattered electron beams 11, 12 pass through an aperture formed in an aperture filter 17, whereas the electrons 13 scattered through larger scattering angles are blocked by the aperture filter 17. After having passed through the aperture filter 17, the electron beams 11, 12 which have been scattered through smaller scattering angles pass through a further projection lens system 19 which focuses the beams 11, 12 for imaging the pattern 9 onto the substrate 3.
The electron field impinging on the mask 1 also comprises beams 21 which do not pass through the spaces between the struts 7, as is the case with the above-described beams 11, 12, 13, but impinge on the struts 7. Due to the thickness of the struts 7, the beams 21 are more likely to be scattered than the beams 11 and 12 which merely pass through the membrane layer 5. Therefore, the regions of the mask which are positioned in its projection along the electron field below the struts are not used to form the pattern. Accordingly, beams 21 which pass through the struts 7 and the membrane layer unscattered shall not pass through the aperture filter 17 onto the substrate 3 either.
For this purpose, European Patent Application Number 0,969,326 A2 proposes a spatially limited electron beam, the cross-section of which in the mask plane is limited such that it passes through the mask at spaces between adjacent struts and does not strike the struts as such. Such spatially limited beam is then moved relative to the mask in the direction into which the struts extend in order to scan all regions of the mask used to form the pattern. In order to spatially define the beam cross-section, European Patent Application Number 0,969,326 A2 proposes, for example, to use aperture filters. This enables the beam to form such that its cross-section corresponds exactly to the inside width between adjacent struts, and scattering of the electrons at the struts is thus preventable, provided that the beam itself is positioned with sufficient accuracy relative to the mask and the scanning motion of the beam relative to the mask is likewise performed with sufficient accuracy so that the field of beams does not strike the struts. Moreover, the regions between the struts of the mask are imaged onto adjacent strips on the substrate to be exposed. Positional inaccuracies in scanning the mask may then result into underexposures or double exposures on the substrate.